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Power Factor Correction

When it comes to improvement, the growth of the organic layers used in OLEDs has been widely unaddressed (22), which is in stark contrast to the attention given to performance optimization of organic photovoltaics. There, an optimized substrate temperature during film growth involving the transition to crystalline order is thought to allow best nano- and macroscopic blend morphologies, which are needed to maximize charge separation and extraction (23). From a different and more fundamental approach, in recent years, there has been an increasing interest in using physical vapor deposition as a powerful strategy to define the properties of amorphous layers of organic molecules (24). By properly setting the deposition conditions, essentially the growth rate and the substrate temperature during deposition, it is possible to achieve glasses with properties that outperform both conventional glasses prepared by quenching the liquid and traditional vapor-deposited molecular glasses grown at temperatures far below the glass transition temperature Tg. The specific properties of the glass are highly dependent on the deposition temperature. Glasses evaporated at substrate temperatures (Tsub) around 85% of Tg (K) and low growth rates, generally below 5 Å/s (25), show higher thermal and kinetic stabilities, lower enthalpies (2628), higher densities (2829), higher elastic moduli (30), lower water uptake (31), and lower expansion coefficient and can also exhibit a certain degree of anisotropy (2932). Glasses obtained under these conditions were dubbed highly stable glasses or ultrastable glasses because their equivalent stability would only be attained after an extended aging (thousands or even millions of years) of a conventional glass. The enhanced surface diffusion at the outer surface of the glass during the deposition process is believed to be at the origin of the observed stability and density enhancement (2433). Understanding of vapor-deposited glasses, allowing the outstanding properties mentioned, has recently been addressed by computational methods (3435)